Searching for oil or more generally hydrocarbons is becoming more demanding in terms of hardware and devices in recent years because oil and gas fields (reservoirs) are located deeper in the earth or in places difficult to reach.
Numerous onshore drilling and production activities require tubular connections having high levels of fatigue resistance; for example, drilling applications and thermal applications.
Additionally, exploring and producing hydrocarbon fields in deep water environments (offshore applications) has increased and necessitates tubular connections which are more resistant to environmental challenges such as fatigue and corrosion.
Off-shore platforms have production facilities located above the sea surface. These facilities are frequently used for exploitation of hydrocarbon fields lying below the sea floor. These platforms are anchored to the sea bottom and tubular strings are used to deliver the hydrocarbons from wells drilled into reservoirs below the sea bed. The tubular strings are sometimes referred to in the art as “risers.”
These riser strings are immerged in the sea and are subject to movements caused by sea currents and surface wave movements. Because of continuous and periodical movements of the sea, the tubular strings do not remain immobile, but are subject to lateral movements of small magnitude which can produce deformations in certain parts of the tubular connections. These riser strings must withstand loads which induce fatigue stresses in the tubes and the tubular connections, in particular with respect to the zone of the threaded connection. These stresses tend to cause ruptures in the tube and/or connection in the vicinity of the thread and there is a need to improve the fatigue resistance of the threaded connections.
Some prior art patents, for example U.S. Pat. No. 7,780,202 and U.S. Pat. No. 6,609,735 disclose flank-to-flank (“FtF”) engagement type connections which are subject to fatigue, including riser connectors.
Other prior art conventional interference fit threaded connections (including API buttress-style thread forms), have profiles in which the threads engage along only one thread flank upon make up. This type of connection must completely unload the contacting flank, undergo relative movement between the pin and the coupling until the opposite flanks contact, and then transfer load to the newly contacting flank. Repeated, cyclical side loading and load transfers make these connection types especially susceptible to fatigue failures.
In flank-to-flank (FtF) threads, upon make up, contact is made between both stabbing and load flanks Clearance exists between crests and roots. The thread is designed with the thread teeth of one member being wider than the mating teeth of the other member (e.g., flank to flank interference). Due to the inclination of the flanks, contact forces (normal to the surface of the flank) have the main component placed in an axial direction, pressing the material that forms the thread teeth. To achieve the flank to flank interference, contact forces work mainly on the elasticity of the teeth. The elasticity of the teeth is very low so high contact pressures are reached during make up. This explains why FtF threads have high galling tendency during make up.
Additional drawbacks of FtF threads are present for very sloping angles of the lead-in flank of the thread, measured compared to a perpendicular surface to the pipe's axis. The compression action of the connection is unsatisfactory because this type of solution aids the onset of the phenomenon defined as “jump-in,” when the compression forces exceed certain limits. Jump-in occurs when the male pipe segment slides into the female segment, exceeding the resistance given by the threading of the two pieces. This phenomenon occurs more frequently the more inclined the angle of thread lead-in.
Other drawbacks of the FtF type of thread is that it is subject to high risk of seizure of the joint with the consequent risk of not ensuring the airtight seal of the fluids inside the tube. Due to the seizure effect, torque varies greatly as the screwing operation (make up) of the joint proceeds. This type of joint typically has more turns. This introduces difficulties in making the joint and creates the possibility of imprecision in applying the correct make up torque.
In Crest-to-Root (CtR) Threads (which are used in the threaded connection of the present disclosure), upon make up, contact is made between a pair of mating flanks (load flanks for tension or stabbing flank for compression) and also contact between crest and roots. The CtR thread is designed with interference between crest and roots. In this case the main component of the contact forces (normal to the surface of the crest/root) are placed in a radial direction, and so the interference is achieved taking advantage of the elasticity of a tubular body by deforming geometrically the pipe. Only a minor part of the interference is achieved by the elasticity of the thread teeth, so contact pressures achieved on the teeth are lower than in the case of FtF threads, and so the galling tendency during make up is diminished.
The CtR design of the present disclosure has an optimum fatigue performance and also a very low galling tendency during make up. Therefore, the presence of micro cracks (due to such galling) is minimized.
The present disclosure can be used in integral connections, threaded and coupled connections and in big outside diameter (“OD”) threaded connectors, for offshore and onshore applications. There are two major types of big OD threaded connectors used for production risers. The first type is referred to in the art as a “welded” type; the pin and box are machined separately from heavy-wall material and then welded to the pipe. In the second type, referred to in the art as “threaded-and-coupled” type, the pin is typically machined directly onto the pipe ends. The box is machined into each end of a coupling that is used to join the pipe ends together.
Moreover, the design of the present disclosure can be combined with internal and/or external/and or intermediate metal to metal seal configurations, internal and external elastomeric seals, intermediate metal to metal seals and two step threads. For big diameter connectors, stabbing guides and anti-rotation devices can also be used together with the thread profile of this disclosure.